Some conventional spiral separation membrane elements are known to have a structure in which one or more separation membranes, one or more feed-side channel components and one or more permeation-side channel components are wrapped around a perforated hollow core tube. In the case of a reverse osmosis membrane, it has been reported that if a rhombic type net channel component is used as a feed-side channel component, the pressure loss can be reduced (for example, see Japanese Patent Application Laid-Open (JP-A) No. H11 (1999)-235520, JP-A No. 2000-000437 and JP-A No. 2000-042378 below). For example, such a structure is as shown in FIG. 11.
In order to reduce the pressure loss of a feed-side channel, a ladder-type net channel component is also employed that is composed of warp yarns parallel to the direction of feed fluid flow and weft yarns interlaced with the warp yarns (for example, see JP-A No. 1-105 (1993)-168869 below). This publication pays no attention to the relationship between the thicknesses or diameters of the warp and the well or the relationship between the warp spacing and the well spacing or discloses nothing about the thickness of the warp or the weft.
In the feed-side channel, however, the resistance to feed water flow significantly depends on the feed-side channel component, and the nature of feed water or components contained in feed water can be a cause of an increase in the resistance depending on the quality of feed water.
In the conventional ladder type net, the weft and the warp are generally the same in diameter, the weft can inhibit the flow of feed fluid, and suspended components can cause blockage of the channel. In the rhombic type net with no differentiation between warp and weft, the yarns in two intersecting directions cross the flow channel so that the same problem can occur. Namely, there is a problem in which components suspended in feed fluid can get snagged on the weft of the feed-side channel component to increase the flow resistance or block the flow, though the feed-side flow channel component is required to have not only the function of making the feed-side pressure loss as small as possible but also the function of facilitating surface regeneration on the membrane surface and suppressing concentration polarization. There is also another problem in which the effective membrane area can be reduced because components suspended in feed fluid can get snagged on the weft of the feed-side channel component and be deposited on the membrane surface. An additional challenge is to reduce the pressure loss of the feed-side channel component for the purpose of reducing the running cost of separation membrane elements.
In many cases, conventional nets are formed by a shear method such that fusion bonding between the warp and the weft can be ensured. The shear method uses dies having a number of nozzle holes that are arranged at two circumferential portions (inner and outer portions) in an extruder such that when warp and weft yarns are extruded from the inner and outer nozzle holes being rotated in opposite directions so as to be fused to each other at intersections, both nozzle holes overlap one another to form a single nozzle hole at the intersection of the warp and weft yarns. In the shear method, the amount of extruded resin becomes high at the intersections of the warp and weft yarns so that these portions are deformed into a web-like form. As a result of investigations, the inventors have found that the web-like form causes an increase in the pressure loss of the feed-side channel.